Properly support subclassing of WireMockExtension

[tomakehurst]

Per this comment: #1439 (comment)

Subclassing the JUnit 4 rule is very useful a) to be able to hook into its before/after lifecycle methods and b) to create API wrappers around specific mock APIs.

WireMockExtension for JUnit 5 should support this too, with the following considerations:

• Lifecycle template methods created as with the JUnit 4 rule (but all 4, rather than 2). These should probably take WireMockRuntimeInfo as a parameter.
• How to handle constructors in a way that avoids a new overload being created each time we add a new parameter.